Carrillo Roas Sara, Yagita Yuichi, Murphy Paul, Kurzbauer Robert, Clausen Tim, Zavodszky Eszter, Hegde Ramanujan S
MRC Laboratory of Molecular Biology, Cambridge CB2 0QH, UK.
Research Institute of Molecular Pathology, Vienna BioCenter, 1030 Vienna, Austria.
Mol Cell. 2025 Feb 20;85(4):815-828.e10. doi: 10.1016/j.molcel.2025.01.002. Epub 2025 Jan 28.
Unassembled and partially assembled subunits of multi-protein complexes have emerged as major quality control clients, particularly under conditions of imbalanced gene expression such as stress, aging, and aneuploidy. The factors and mechanisms that eliminate such orphan subunits to maintain protein homeostasis are incompletely defined. Here, we show that the UBR4-KCMF1 ubiquitin ligase complex is required for the efficient degradation of multiple unrelated orphan subunits from the chaperonin, proteasome cap, proteasome core, and a protein targeting complex. Epistasis analysis in cells and reconstitution studies in vitro show that the UBR4-KCMF1 complex acts downstream of a priming ubiquitin ligase that first mono-ubiquitinates orphans. UBR4 recognizes both the orphan and its mono-ubiquitin and builds a K48-linked poly-ubiquitin degradation signal. The discovery of a convergence point for multiple quality control pathways may explain why aneuploid cells are especially sensitive to loss of UBR4 or KCMF1 and identifies a potential vulnerability across many cancers.
多蛋白复合物的未组装和部分组装亚基已成为主要的质量控制对象，尤其是在基因表达失衡的情况下，如应激、衰老和非整倍体状态。消除此类孤儿亚基以维持蛋白质稳态的因素和机制尚未完全明确。在此，我们表明，UBR4-KCMF1泛素连接酶复合物是从伴侣蛋白、蛋白酶体帽、蛋白酶体核心和蛋白质靶向复合物中有效降解多个不相关孤儿亚基所必需的。细胞中的上位性分析和体外重组研究表明，UBR4-KCMF1复合物在引发泛素连接酶下游起作用，该引发泛素连接酶首先将孤儿亚基单泛素化。UBR4识别孤儿亚基及其单泛素，并构建K48连接的多泛素降解信号。多个质量控制途径汇聚点的发现可能解释了为什么非整倍体细胞对UBR4或KCMF1的缺失特别敏感，并确定了许多癌症中的一个潜在脆弱点。
